1. Field of the Invention
The present invention is related to an incinerator for burning waste material and, more particularly, to an apparatus for destroying small quantities of toxic orgainc substances which may be present in the exhaust produced by the incineration of municipal solid waste.
2. Description of the Related Art
Proper disposal of solid waste has become an increasingly serious problem as existing sites for land disposal near capacity and new sites become increasingly difficult to locate, while the amount of municipal waste appears to be increasing. Incineration of combustible solid waste has long been used to recduce the quantity of solid matter needing disposal. However, in the last few decades it has become widely known that when municipal solid waste is incinerated, trace amounts of toxic chemicals such as dioxins and furans, among others, may be discharged in the exhaust.
Within the last two decades, studies by the U.S. Environmental Protection Agency, environmental protection agencies in foreign countries, and other groups have determined that such toxic chemicals are broken down at temperatures ranging between 600.degree. C. to slightly over 1300.degree. C. for an exposure period lasting somewhere between one-tenth (0.1) to two and one-half (2.5) seconds. The most recent studies indicate that over 99 percent of some substances, such as biphenyl, dibenzofuran, dibenzo-p-dioxin and tetrachlorobiphenyl, are destroyed at temperatures of slightly over 700.degree. C. with a residence time (exposure period) of approximately one second. These values are based on a study by Duvall and Rubey, conducted in 1977 and published in "Laboratory Evaluation of High-Temperature Destruction of Polychlorinated Biphenyls and Related Compounds", U.S. EPA Report EPA-600-2-77-228. More recent studies including one conducted for the Danish National Environmental Protection Agency with results published in 1984 under the title "`MILJO-RAPPORT` Bilag rapporten om Dannelse og spredning of Dioxiner isaer i forbindindelse med affaldsforbraending", translated as "Formation and Dispersion of Dioxins, Particularly in Connection with Combustion of Refuse", indicate that exposing exhaust gases to a temperature of approximately 1000.degree. C. for a period of at least two seconds results in destruction of essentially all dioxins and furans.
The results of these studies appear contradictory to practical experience, in that temperatures well above 1000.degree. C. are commonly reached in the process of incinerating municipal solid waste, yet it is known that potentially toxic materials are present in the exhaust from municipal waste incinerators. To elucidate the nature, or source, of this problem, reference is had to FIGS. 1A to 1C, which illustrate a type of incinerator in common use, known as a water-cooled rotary combustor; FIG. 1A also schematically illustrates the incorporation therein of the present invention, discussion of which is deferred to a later section of this disclosure.
The water-cooled rotary combustor of the foregoing figures comprises a combustion barrel 10 having a generally cylindrical side wall 23 formed of longitudinally extending cooling pipes 24 and gas-porous interconnections 36, such as perforated webs (FIG. 1A illustrating only a few such webs 36 between adjacent cooling pipes 24). The combustion barrel 10 has a central axis of rotation which is inclined slightly from the horizontal, proceeding downwardly from an input end 16 to an exit 18. Thus, the cooling pipes 24 and perforated webs 31 are also slightly inclined from the input end 16 until the pipes 24 bend inside the flue 28. The cooling pipes 24 have first and second ends disposed adjacent the exit end 18 and input end 16, respectively, of the barrel 10. Combustion typically is initiated in the barrel 10 by using an auxiliary fuel, such as oil or natural gas, which can be supplied through the input end 16 of the combustion barrel 10, as disclosed in Harris et al. '651.
The perforated webs 36 are preferably formed of bar steel having openings 37 therein, for supplying combustion air to the interior of the combustion barrel 10. The webs 36 extend from the input end 16 and along the generally straight axial portions of the pipes 24 to an angled section 24a inside the flue 28. No webs 36 are included after the angled section 24a, in which the cooling pipes 24 extend in a somewhat converging relationship to the exit end 18 of the barrel 10, permitting exhaust 20, including exhaust gases and solid particles such as fly ash, and solid combustion products 22, e.g., ash and cinders, to escape more easily from the barrel 10.
As illustrated in FIG. 1A, the cooling pipes 24 are affixed to annular support bands 13 which are received on rollers 12. The rollers 12 may be driven to rotate the barrel 10 about its longitudinal axis, or a separate ring gear (not shown) may be attached to the side wall 23 and driven by a pinion, as disclosed in U.S. Pat. No. 3,822,651 to Harris et al., incorporated herein by reference.
As noted above, high temperatures in excess of 1000.degree. C. are reached in the combustion barrel 10, waste 14 being input at the generally open input end 16 and incinerated by air supplied through windboxes 38. The combustion barrel 10 is able to withstand such high temperatures because coolant is circulated through the cooling pipes 24 and discharged from the barrel 10 via a ring header 17 and supply pipes 26. High-energy coolant discharged by the supply pipes 26 is circulated by a pump 32 through a rotary joint 30, such as the joint disclosed in Harris et al. '651, to heat exchanging equipment 34 which returns low-energy coolant to the ring header 17 via the pump 32, joint 30 and supply pipes 26. The ring header 17 distributes the low-energy coolant to a first set of the cooling pipes 24 which transport the coolant the length of the barrel 10 to return means, such as U-tubes 39 at the input end 16 of the barrel 10. The U-tubes 39 couple the first set of cooling pipes 24 to a second set of cooling pipes 24 which return the coolant to the ring header 17 to be discharged to the heat exchanging equipment 34. The heat exchanging equipment 34 may include a boiler 40 (FIG. 1C), a condenser, connection to a steam driven electrical power generation system, etc., (all not shown) as known in the art.
As illustrated in FIG. 2, air is supplied to the combustion drum 10 by a air duct 30 which is connected to the windboxes 38 via individual control ducts 35. There are a total of six windboxes 38 disposed under the combustion barrel 10. The windboxes 38 are arranged in three zones from the input end 16 to the exit end 18, as illustrated in FIG. 1A, with one underfire windbox 38.sub.u and one overfire windbox 38.sub.o in each zone, as illustrated in FIG. 2. A cut-away 59 is provided in FIG. 2 to illustrate the connection of underfire windbox 38u to control duct 35u. The windboxes 38 receive the combustion air under pressure from a blower (not shown). The pressure is maintained by seal strips 54 which extend longitudinally along the exterior of the combustion barrel 10 and have a dog leg-shaped cross-section, as illustrated in FIG. 2. The seal strips 54 are continuous for at least the axial length of one windbox 38 and help form a pressure seal against windbox edges 56 so that the combustion air exiting the windboxs 38 enters the combustion barrel 10.
An enclosure 57, illustrated in FIG. 2, but excluded from FIG. 1A to simplify the drawing, is supported on a suitable surface by supports 58. The enclosure 57 surrounds the side of the barrel 10 so that combustion air is pulled from the input end 16 and windboxes 38 by an induced draft fan 50 (FIG. 1C). The induced draft fan 50 is coupled to the flue 28 downstream from the rotary combustor to maintain the flue 28 at slightly below atmospheric pressure. Thus, essentially all exhaust gases in the exhaust 20 exit from the combustion barrel 10 via the flue 28.
The above-described rotary combustor produces gas temperatures in the exhaust gases of up to 1100.degree. C. However, the side wall 23 of the combustion barrel 10 is maintained at a temperature of approximately 275.degree. C. by the circulated coolant. The walls of the flue 28 are typically lined with boiler tubes (not shown) coupled to the heat exchanging equipment 34 and thus, are maintained at approximately 275.degree. C. However, the heat distribution in municipal solid waste incinerators is so uneven that not every molecule of exhaust gas is exposed to 1000.degree. C. for a sufficient period of time to destroy trace organic substances such as dioxins and furans.
A conventional exhaust cleaning system 41 for a rotary combustor is illustrated in FIG. 1C. In a typical rotary combustor installation, the coolant output from the combustion barrel 10 is supplied via pipes 33a and 33b to a boiler 40 prior to being supplied to other elements of heat exchanging equipment 34. The boiler 40 extracts heat from the high temperature exhaust 42 and transfers the heat to the coolant. Thus, the exhaust 44 output from the boiler 40 is considerably lower in temperature than the exhaust 42, as a result of passing through the boiler 40. The reduced temperature exhaust 44 passes through a cyclone separator 46 for removing larger particles and is passed through a gas clean-up station 48 which typically is a conventional "baghouse" or an electrostatic precipitator. An induced draft fan 50 aids in the flow of the exhaust 42 and 44 through the boiler 40, cyclone separator 46 and gas clean-up station 48, and finally the reduced temperature exhaust 44 is discharged as emissions 53 from a discharge port 51 in a stack 52.
Conventionally, when additional gas clean-up, beyond that provided by the cyclone 46 and the baghouse station 48, is desired, additional elements, such as electrostatic precipitators, scrubbers, or catalysts, are added to the gas clean-up station 48 or in, or on top of, the stack 52. None of these known devices has been shown to function efficiently for removing organic substances. Furthermore, due to the relatively low temperature of the exhaust gases at these points, considerable external sources of heat are required if toxic organic substances are to be destroyed by heating in the clean-up station 48 or stack 52. In addition, postponing the removal of solids until after passage of the exhaust through the boiler 40 results in formation of deposits in the boiler 40 degrading heat transfer from the exhaust to coolant routed through the boiler 40.